bug#17453: Isearch doesn't work properly with Follow Mode.

[Eli Zaretskii]

> Date: Sat, 7 Nov 2015 12:59:31 +0000
> Cc: Artur Malabarba <bruce.connor.am@gmail.com>, Eli Zaretskii <eliz@gnu.org>,
>   17453@debbugs.gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> Proposed solutions:
> 1. Call follow-post-command-hook from isearch-update before calling
>   isearch-lazy-highlight-new-loop (as described above).
> 2. Call the proposed function `redisplay-would-scroll-window' instead of
>   the `sit-for'.
> 3. Make isearch-lazy-highlight-new-loop always set the idle timer, and
>   test for the need for a new loop instead in the function it triggers.
>   Remove the `sit-for'.
> 
> I now think solution 2. is not sensible or realistic.  Redisplay is just
> too complicated to second-guess.
> 
> Solution 1. has the disadvantage that follow-post-command-hook would be
> called twice for every command in Isearch.  It is not fast.
> 
> Solution 3. similarly might have the problem that if
> lazy-highlight-initial-delay is set to zero, redisplay might not have
> done its work when isearch-lazy-highlight-update runs.  (I haven't tried
> it out, yet).
> 
> Personally, I am in favour of solution 3, but I'm willing to be persuaded
> into solution 1.  But I'd like us to come to a decision quickly.

How about using 1), but also adding some indication that could prevent
the post-command-hook from being called twice?